Envelope tracking (ET) is a key feature of a user equipment (UE) to reduce energy consumption for radio transmission. ET is expected to be of even more importance for future high-end UEs. To meet an increasing demand for data throughput, complex modulation schemes, e.g., Orthogonal Frequency Division Multiplexing (OFDM), are applied that cause a largely varying signal envelope in a UE transmitter. Power amplifiers (PAs) operating with a constant supply voltage may therefore spend much time operating below their peak efficiency. An ET system continuously adjusts the power supply voltage of the PA to the amplitude of the signal to be amplified, so that the amplifier can permanently operate at peak efficiency. The ET system thus contributes significantly to reducing the energy consumption of the PA in the UE transmitter by minimizing the energy wasted by the PA through heat dissipation.
Envelope tracking is a nonlinear control approach. However, the signal transmission chain has to provide sufficient linearity, e.g., for fulfilling requirements on the Adjacent Channel Leakage Ratio (ACLR) and Error Vector Magnitude (EVM). Therefore, an accurate calibration is required to align the nonlinearities in the ET system such that an overall linear gain is achieved. A critical part of the calibration is the time alignment (TA) of the transmitted signal and the respective envelope. In currently known ET systems, however, TA calibration is very time consuming. Additionally, significant UE resources for computation and memory are required to obtain and store calibration coefficients. Therefore, solutions have been searched to reduce time and resources required for TA calibration.
In other known ET systems, the TA calibration is performed during UE production. However, TA changes due to component aging or parameter drift, e.g. temperature variation, which cannot be considered at the time of production. Therefore, efforts have been made to avoid TA calibration at production by implementing a closed-loop control for TA during normal operation of the UE. A closed-loop control additionally allows compensating variations of the antenna voltage standing wave ratio (VSWR) during TA, which can otherwise cause ACLR and EVM degradations.
Conventional closed-loop approaches for TA are disclosed in documents US 2013/0235949 A1 and US 2011/0274210 A1. However, the methods presented in the prior art have shortcomings in their practical application. The document US 2013/0235949 A1 uses a numerically demanding algorithm for calculating the TA error, including two Fast Fourier Transforms and several complex number calculations. Furthermore, the method for deducing the TA depends on peak and minima detections, which is sensitive to noise. The further document US 2011/0274210 A1 requires many iterations in the control loop for finding the optimum TA, thus severely limiting efficiency with respect to time. Also, the U-shaped cost function for determining the optimum TA does not indicate whether the TA error is positive or negative. Moreover, the function cannot be sharply evaluated and adds susceptibility to noise.